It is well known in the art that a laser beam can be modulated using non-centrosymmetric electro-optic single crystal materials, such as CdTe (i.e., materials that exhibit a linear electro-optic effect). When an electric field is applied to the crystal, internal birefringence is created that causes a rotation of the polarization direction of the crystal material. When the crystal is positioned between a polarizing element and an optical analyzer, the amplitude of the exiting beam is proportional to the electric field applied to the crystal. Thus, modulation of the applied electric field (such as by pulsing the voltage applied across the crystal) provides a modulated optical output signal.
In practice, conductive layers are applied to the crystal, which are then contacted by the electrodes used to apply the voltage across the crystal. The conductive layers are utilized to out-couple heat and acoustic energy away from the crystal, while also providing the necessary electrical contact between the high voltage source and the surface of the crystal.
It is known that the voltages used to create the electric field within the crystal may exceed upwards of 3000V (depending on the design thickness of the crystal), which can produce undesirable arcing and corona that may ultimately destroy the crystal. For example, presuming that a crystal having a thickness of 5 mm is used as the modulator, an applied voltage on the order of 6000 volts (1200 volts/mm) is required to produce a full rotation of the polarization state within the crystal. Experience has shown that corona will form at this electric field intensity, eventually leading to arcing. Once the voltage exceeds about 7000 volts, arcing will occur instantly. Corona is known to be corrosive in nature, as a result of its high discharge temperature and formation of ozone and, therefore, a source of damage to the modulator structure.
While many “high voltage” electronic circuits may be fully immersed in a potting material to prevent corona formation and arcing, that is not possible when using birefringent elements such as crystal, material CdTe, since the various mechanical stresses associated with the potting process creates an unknown, amount of stress-induced birefringence within the crystal. This variable (and uncontrollable) amount of polarization shifting significantly degrades the ability of the configuration to operate as an optical modulator.
Thus, a need remains in the art for an electro-optic modulator that is protected from arcing and corona problems.